The prospect a faster-than-light spacecraft is incredibly tempting. But is it practical? Do the laws of physics really allow us to travel to other systems in a human lifetime? In this week's "Ask a Physicist," we'll find out.

My last column, which asked, "Can you surf a gravitational wave?", got some people excited about traveling at faster than light speeds, when in reality it was about whether (still quite awesome) gravitational waves were real and detectible.

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Although it's not really a gravity wave, I think it would be cool if Dave did a column on the Alcubierre space warp.

A much better way to ask your questions is to send them to me directly, but Mr. Metal raises a good point. You've heard about ideas like Alcubierre drives or wormholes, or other mechanisms for traveling faster than light. Are these just engineering problems waiting to be solved, or is there some fundamental limitation preventing us from breaking the light barrier?

First, the obvious. You're probably already familiar with the idea that the speed of light, about 300,000 km/s, is supposed to be the ultimate speed limit in the universe, and I've already done a column (the first of this series, in fact), explaining why massive things (like a spaceship) can't ever reach the speed of light, let alone exceed it.

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That isn't to say that traveling to a star 20 light-years away will necessarily seem to take more than 20 years. There are some ways to cheat. Relativity shows that time is slowed for somebody moving close to the speed of light. Travel fast enough, and it's possible you'll age that far less than 20 years.

A while back, I did a calculation in my blog in which I found that it might only take about 6.1 years (according to the astronaut on the ship) to reach Gliese 581g, an exoplanet about 20.5 light years away. Ta da! Even though the top speed is only 99.6% c, it seems as though the astronaut opened up a can of whoop ass on the speed of light. A moment's sober reflection (and checking in with the folks back home) reveal the truth. The trip actually took 22.4 — decidedly longer than it would have taken to shoot a laser at the planet.

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The quest to travel faster than light could fill a whole book, but frankly, most of it would be insanely speculative. Responding to queries on facebook and twitter, people asked about things like tachyons or shortcuts through higher dimensions, neither of which we have any good reason to believe actually exist, let alone how we'd manipulate into a spaceship.

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My list of "good" FTL devices is almost certainly not complete, so you should feel free to present and debate other options in the comments section. But for now, let's start with the suggestion that prompted the whole discussion:

Alcubierre Drive

How does it work?

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General relativity gives us some good options for tinkering with time and space, based on how much matter and energy are in different parts of the universe. In 1994, Miguel Alcubierre came up with a very interesting solution to Einstein's Field Equations of general relativity. It is essentially a bubble that propagated through space at arbitrarily high speeds, while a spaceship inside the bubble would feel like it's in free-fall.

I'm going to be a bit of a downer in this discussion, but that can wait for a bit. The good news is that the Alcubierre drive is a gen-u-ine solution to general relativity. Even better, since the astronaut isn't accelerating within the bubble, there's no time dilation effect from simply making the trip to another planet. This is good news if you'd like to return and not have all of your romantic prospects aged out of the "half plus seven" rule.

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As seen in...

Think of the warp drive on Star Trek, and you've basically got the right idea, though both The Original Series, and The Next Generation predate Miguel Alcubierre's solution.

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There are some complications, however. Normally, if you accelerate your spaceship, you'll be pushed to the back, essentially creating artificial gravity. In the Alcubierre drive, on the other hand, even an accelerating ship won't feel like it's accelerating, so no artificial gravity. Since most sci-fi ignores this problem anyway, it's not a huge problem.

So what's the problem?

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Even under the best of circumstances, it's not clear how you'd get into the Alcubierre bubble to start your trip. While the interior of the bubble behaves like flat space (no acceleration), the edges are another matter entirely. The transition from inside to outside (or the other way around) would produce immense tidal forces, potentially destroying your ship.

Secondly, setting up an Alcubierre drive might be impossible, and destroying it, perhaps equally difficult. I don't want to get into the equations here, but Alcubierre's solution essentially describes a deformation of spacetime going eternally into the past and forever into the future. It's not clear how this could be set up in the first place, or, assuming you could escape the bubble, how to destroy it before it propagates into some other system. Even if you ignore the runaway bubble as somebody else's problem, there's still the matter of setting the thing up in the first place.

All solutions in general relativity require manipulation of matter and energy, and one of the problems with the Alcubierre drive is it requires a negative energy density. This is an issue since it's not obvious that so-called "exotic energy" does exist, or can exist in our universe.

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Anticipating sophisticated objections from the io9 audience, someone is likely to bring up the similar idea of the "Casimir Effect,", which, among much else, shows that the vacuum of space seems to be permeated by a vacuum energy with some very weird properties. But vacuum energy and exotic energy are very different things. Vacuum energy has a negative pressure, which is strange enough, but exotic energy actually requires a negative energy density. The other difference is that we're pretty sure that vacuum energy exists (since we can detect it using the Casimir Effect). With exotic energy, we have no such assurance.

Finally, even if all of this weren't enough, Alcubierre drives may be impossible from a practical perspective. Suppose you were to actually construct a drive capable of crossing the galaxy. According to reasonable estimates, it would take more than the mass of the entire observable universe converted into fuel to launch a spaceship-sized bubble.

Wormholes

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How does it work?

I've talked a bit about the problems with wormholes in my previous column on time travel, but you you might suppose that the problems of traveling through space are trivial compared to those of traveling through time. As a reminder, a wormhole has two mouths, each of which have a similar gravitational field to a black hole when seen from far away.

Closer in, however, there is a throat connecting the mouths, inside of which space appears more or less flat. A wormhole can act as a "permanent" two-way connection between two points and the travel time between the two mouths is literally only the amount of time that it takes to traverse the throat of the wormhole. Like the Alcubierre drive, wormholes are a perfectly legit solution to the Einstein Field Equations.

We have no idea how to create one. One way of thinking of wormholes is that they are a rip in spacetime, and while there's some speculation that wormholes form on microscopic scales constantly, we don't have any idea how to either make a macroscopic one from scratch or grow a microscopic wormhole until it's a macroscopic one.

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But even supposing that you could make one, it's not clear how to keep it from collapsing. The original wormhole, the one described by Einstein and his collaborator Nathan Rosen, was so unstable that before a single photon could pass through it, the entire thing would collapse.

Modern versions, like those proposed by Kip Thorne and his collaborators are much more stable, but require — you guessed it — exotic matter to hold them open. No exotic matter, no wormholes.

There's another complication. Even if you could make the wormhole in the first place, you'd need to drag one of the mouths to where you'd like to go which requires, at least in setting it up, making the trip at slower than the speed of light. At least in part, that kind of defeats the purpose.

Quantum "Teleportation"

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How does it work?

Not every FTL mechanism requires relativity. Some are straight-up applications of quantum uncertainty. I've covered quantum teleportation in a previous column. The idea is that you can use the entanglement of particles to take the state of one particle and "instantaneously" send the information to a distant particle.

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If you were transport a redshirt from the surface of a planet to your ship, his physical atoms would be left on the planet, while his pattern would be recreated on the ship. However, as I discussed in a previous column, in order to correctly reconstruct him, you need to send a signal at less than or equal to the speed of light from the planet.

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But there's another possibility which is more properly called quantum tunnelling than teleportation (hence the quotes above). The idea is that your position has an uncertainty, so it's technically possible (however unlikely) that you could instantly be detected at some distant point in space. Or, to put it another way, that you could teleport there — instantly.

As seen in...

Quantum teleportation (or something similar) shows up the Star Trek transporter device, of course, but exploiting uncertainty shows up most obviously, in the "Heart of Gold," in the Hitchiker's Guide to the Galaxy and Asimov's Foundation series. I've also always assumed that the so-called jump drives in Battlestar Galactica used this technology.

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So what's problem?

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You can't steer. Come to that, you can't even decide to teleport in the first place. The whole magic of quantum mechanics in this context is that it's an entirely random process. It's possible that at some instant you randomly teleport light-years away, but the probability becomes vanishingly small.

To give you some idea of how what I mean by "vanishingly small," a human being is likely to hop around at distances less than a quintillionth the size of an atomic nucleus. The probability drops off exponentially with distance, so while you could teleport across the Galaxy, you probably won't. That's why you need an infinite improbability drive to do so. Good thing, too, since there's a much better than even chance that you'll end up in deep space rather than on a class M planet.